In the proposed program we will develop an instrument for measuring small amounts of transient birefringence in microscopic biological samples, and will apply it to an existing measurement requirement in the area of neurophysiology. The proposed Laser Micropolarimeter will be capable of measuring sample birefringence with sensitivity at least 100 times greater than that of currently available techniques, and with spatial and temporal resolutions of 2 microns (diameter) and 1 msec respectively. The basis for the major improvement in instrument sensitivity is the application of a stabilized 2-frequency helium neon laser with a heterodyne signal analysis system. This approach directly measures birefringent-induced phase changes in light frequency, circumventing the limitations inherent in the conventional technique which relies upon intensity measurements obtained through crossed polarizers. In phase I, our prototype instrument will be used in voltage-clamp experiments to measure the induced membrane birefringence response of the squid axon. Results will be compared with those obtained by conventional methods. Since transient membrane birefringence changes have been associated with neuron action potentials and muscle fiber excitation, there is a community of investigators who will benefit immediately from the proposed project. Birefringence measurements offer an attractive non-invasive alternative to microelectrode methods for membrane potential studies. With a more sensitive and more practical technique for minute polarization measurements, we foresee many applications in cell biology and diagnostic pathology.